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BQ4850Y Datasheet(PDF) 5 Page - Texas Instruments |
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BQ4850Y Datasheet(HTML) 5 Page - Texas Instruments |
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5 / 17 page  Calibrating the Clock The bq4850Y real-time clock is driven by a quartz con- trolled oscillator with a nominal frequency of 32,768 Hz. The quartz crystal is contained within the bq4850Y package along with the battery. The clock accuracy of the bq4850Y module is tested to be within 20ppm or about 1 minute per month at 25°C. The oscillation rates of crystals change with temperature as Figure 3 shows. To compensate for the frequency shift, the bq4850Y of- fers onboard software clock calibration. The user can adjust the calibration based on the typical operating temperature of individual applications. The software calibration bits are located in the control register. Bits D0–D4 control the magnitude of correc- tion, and bit D5 the direction (positive or negative) of correction. Assuming that the oscillator is running at exactly 32,786 Hz, each calibration step of D0–D4 ad- justs the clock rate by +4.068 ppm (+10.7 seconds per month) or -2.034 ppm (-5.35 seconds per month) depend- ing on the value of the sign bit D5. When the sign bit is 1, positive adjustment occurs; a 0 activates negative ad- justment. The total range of clock calibration is +5.5 or -2.75 minutes per month. Two methods can be used to ascertain how much cali- bration a given bq4850Y may require in a system. The first involves simply setting the clock, letting it run for a month, and then comparing the time to an accurate known reference like WWV radio broadcasts. Based on the variation to the standard, the end user can adjust the clock to match the system’s environment even after the product is packaged in a non-serviceable enclosure. The only requirement is a utility that allows the end user to access the calibration bits in the control register. The second approach uses a bq4850Y test mode. When the frequency test mode enable bit FTE in the days reg- ister is set to a 1, and the oscillator is running at exactly 32,768 Hz, the LSB of the seconds register toggles at 512 Hz. Any deviation from 512 Hz indicates the degree and direction of oscillator frequency shift at the test temperature. For example, a reading of 512.01024 Hz indicates a (1E6 ∗0.01024)/512 or +20 ppm oscillator fre- quency error, requiring ten steps of negative calibration (10∗-2.034 or -20.34) or 001010 to be loaded into the cali- bration byte for correction. To read the test frequency, the bq4850Y must be selected and held in an extended read of the seconds register, location 7FFF9, without having the read bit set. The frequency appears on DQ0. The FTE bit must be set using the write bit control. The FTE bit must be reset to 0 for normal clock operation to resume. 5 bq4850Y SLUS057A- January 2005 -120 -100 -80 -60 -40 -20 0 -30 -20 -10 0 10 20 30 40 50 60 70 Temperature ( C) GR485001 Figure 3. Frequency Error Not Recommended For New Designs |
Similar Part No. - BQ4850Y_14 |
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Similar Description - BQ4850Y_14 |
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